This invention relates to vehicle detector systems used to detect the presence or absence of a motor vehicle over an inductive loop embedded in the pavement. More particularly, this invention relates to a vehicle detector system capable of detecting both motorized vehicles and bicycles for purposes of traffic control and for distinguishing bicycles from motorized vehicles.
Vehicle detectors have been used for a substantial period of time to generate information specifying the presence or absence of a vehicle at a particular location sometimes termed a detection zone. Such detectors have been used at intersections, for example, to supply information used by an associated traffic control unit to control the operation of the traffic signal heads, and have also been used to supply control information used in conjunction with automatic entrance and exit gates in parking lots, garages and buildings. A widely used type of vehicle detector employs the principle of period shift measurement in order to determine the presence or absence of a vehicle in or adjacent to the inductive loop mounted on or in a roadway. In such systems, a first oscillator, which typically operates in the range from about 20 kHZ to about 100 kHZ is used to produce a periodic signal in a vehicle detector loop. A second oscillator operating at a much higher frequency is commonly used to generate a sample count signal over a fixed number of loop cycles. The relatively high frequency count signal is typically used to increment a counter, which stores a number corresponding to the sample count at the end of the fixed number of loop cycles. This sample count is compared with a reference count stored in another counter and representative of a previous count in order to determine whether a vehicle has entered or departed the region of the loop in the time period between the previous sample count and the present sample count.
The initial reference value is obtained from one or more initial sample counts and stored in a reference counter. Thereafter, successive sample counts are obtained on a periodic basis, and compared with the reference count. If the two values are essentially equal, the condition of the loop remains unchanged, i.e., a vehicle has not entered or departed the loop. However, if the two numbers differ by at least a threshold amount in a first direction (termed the Call direction), the condition of the loop has changed and may signify that a vehicle has entered the loop. More specifically, in a system in which the sample count has decreased and the sample count has a numerical value less than the reference count by at least a threshold magnitude, this change signifies that the period of the loop signal has decreased (since fewer counts were accumulated during the fixed number of loop cycles), which in turn indicates that the frequency of the loop signal has increased, usually due to the presence of a vehicle in or near the loop. When these conditions exist, the vehicle detector generates a signal termed a Call signal indicating the presence of a vehicle in the loop.
Correspondingly, if the two numbers differ by less than a second threshold amount in a second direction (termed the No Call direction), this condition indicates that a vehicle which was formerly located in or near the loop has departed the detection zone. When this condition occurs, a previously generated Call signal is dropped.
The difference between a sample count N and a reference count R is representative of the inductance change in a loop circuit at the end of the time period between the former sample count (the reference count R) and the current sample count N. More particularly, the quantity ΔL/L=kΔN/N, where L=loop inductance and k is a scaling factor, expresses the relationship between numerical counts and loop inductance.
The Call signal can be either a pulse signal or a presence signal. A pulse signal is a fixed length pulse generated when the vehicle is detected in the loop. A presence signal is a signal which continually persists so long as the vehicle remains in the loop. Some vehicle detectors are provided with a presence/pulse selection feature, which causes the vehicle detector to generate one of these two types of Call signals.
Call signals are used in a wide variety of applications, including vehicle counting along a roadway or through a parking entrance or exit, vehicle speed between preselected points along a roadway, vehicle presence at an intersection controlled by a traffic control light system, or in a parking stall, and numerous other applications.
In addition to the basic function of generating and dropping a Call signal, existing vehicle detectors incorporate other features, some of which are selectable on-site by a technician. For example, some vehicle detectors incorporate an end of green function which requires the detector to automatically reset after the green traffic signal, which controls the lane in which the loop associated with the vehicle detector is located, terminates. Some vehicle detectors are provided with an extension time feature which extends the Call signal for a period of time after a vehicle leaves the associated loop (typically in order to permit ample minimum time for a vehicle to clear an intersection). Some vehicle detectors are also provided with a presence/pulse selection feature, which causes the vehicle detector to generate one of two types of Call signals: a continually persisting signal so long as the vehicle remains in the loop (the presence function); or a fixed length pulse generated when the vehicle is first detected in the loop, or when the vehicle departs the loop (the pulse function). Still other vehicle detectors are provided with selectable different sensitivity settings, which enable a technician to adjust the response of the vehicle detector when connected to the loop in order to accommodate a range of detection conditions.
In the past, vehicle detectors have been designed as either single channel or multiple channel detectors. A single channel detector is designed and configured to operate with only a single loop zone; while a multiple channel vehicle detector is designed and configured to operate with two or more independent loop zones. Multiple channel detectors are designed to be either scanning or non-scanning detectors. A scanning detector operates by sampling only one loop channel at a time, shutting down the active loop, sampling the next loop channel, shutting down that loop, etc. Scanning detectors are typically used in installations in which the probability of cross-talk between loop circuits is more than minimal. Cross talk results when physically adjacent loops are operating at, or near, the same frequency. Cross talk is minimized or eliminated by operating physically adjacent loops on different frequencies. Non-scanning vehicle detectors are configured and function to monitor each of the multiple loop zones simultaneously. Non-scanning detectors are typically used in installations in which there is a very low or no possibility of cross-talk between the multiple loop circuits, such as installations at which the loops are physically separated by a distance sufficient to ensure no overlapping or inter-coupling between the electrical fields associated with the loops. An example of a vehicle detector incorporating the functions described above is disclosed in U.S. Pat. No. 6,087,964 issued Jul. 11, 2000 for “Vehicle Detector With Operational Display”, the disclosure of which is hereby incorporated by reference.
When deployed in an intersection controlled by a traffic control light system, vehicle detectors generate signals which are used by the intersection traffic controller to supervise the operational states of the traffic control heads in response to the arrival and departure of vehicles over loops installed in the various lanes leading to the intersection. One of the key parameters required for the orderly progression of vehicles through an intersection is the clearance time provided for motorized vehicles present at the intersection. In known traffic control systems, the clearance time value is usually selected to allow a motorized vehicle (i.e. an auto, truck, or motorcycle) sufficient time to safely proceed through an intersection when a green phase is presented to a vehicle, without unnecessarily lengthening the duration of the green phase. While this technique works well for motorized vehicles, bicycles present a problem due to the fact that a bicycle typically requires a longer period of time to safely proceed through an intersection than a motorized vehicle. While this problem can be addressed by simply lengthening the period of the initial time and the extension time for all vehicles, this solution is not satisfactory since it inordinately lengthens the duration of the green phase, regardless of whether or not a bicycle is present at an intersection waiting for the green phase to proceed.
Efforts prior to this invention to accommodate the longer intersection clearance time required for bicycles without compromising the efficiency of normal traffic flow have not met with success.